1. Field of the Invention
The present invention relates to an engine having a balancer shaft disposed in parallel with a crankshaft.
2. Description of the Related Art
Prior engines have employed a balancer device, which includes a balancer shaft and balancer weights, to suppress vibration due to the reciprocating motion of pistons and connecting rods in an engine. Japanese Patent Publication No. JP-A-2003-56647 discloses an example of such a balancer device. In the disclosed balancer device, the balancer shaft is journalled at its both ends through bearings disposed within a crankcase and orientated in parallel with a crankshaft. The balancer weights mounted on the balancer shaft on axially outer sides relative to the bearings.
With the conventional balancer device described above, however, a relatively large bending moment acts on bearing portions of the balancer shaft as the balancer weights are mounted at outer ends of the balancer shaft, which project beyond the bearings in a cantilever manner. Therefore, the balancer shaft needs to have a sufficiently large diameter size to endure the bending moment. In addition, since the bearings receive an offset load due to the balancer weights located on axially outer sides relative thereto, the service life of the bearings is relatively short.
A need therefore exists for an engine in which the bending moment acting on bearing portions of the balancer shaft and the offset loading acting on bearings is reduced, and in which upsizing of the diameter of the balancer shaft can be avoided and the service life of the bearings can be extended.